A telephone line circuit is a well known basic part of a telephone network which provides the necessary interface between a subscriber's set and the switching equipment at central switching points such as PBXs or central offices. More specifically, a telephone line circuit performs those supervisory and control functions that are incident to the establishment and maintenance of a connection between a central switching point and one or more local subscribers' telephone sets. One such control function is the hold function where a line is kept in an inactive but waiting condition.
A hold condition is activated by a subscriber pressing the hold button on the telephone set. The establishment of a hold condition requires particular signal conditions on both the communication pair and the A-lead extending between the line circuit and the subscriber's telephone set. Typically a hold condition consists of a non-ground (disconnect) signal on the A-lead together with a telephone connection signal as evidenced by current on the communication pair (tip and ring leads). Similarly, signal conditions on both the communication pair and on the A-lead are also required to establish a telephone on-hook or disconnect condition. In contrast to the establishment of a hold condition, however, a telephone on-hook condition requires a non-ground (disconnect) signal on the A-lead, together with the absence of current (disconnect) signal on the communication pair.
Under certain circumstances battery from a central office may temporarily be removed from the tip (T) and ring (R) leads. Such a condition is commonly referred to as an open switching interval (OSI) which may last up to 460 milliseconds. These OSI conditions typically occur within about one half of a second after an incoming call is answered. A subscriber attempt to establish a hold condition during the OSI period results in the inadvertent dropping or disconnection of the telephone call. A disconnection results because the conditions for a telephone station on-hook condition are satisfied. When the subscriber presses the hold button a non-ground signal is established on the A-lead. This condition together with the OSI caused absence of current on the T and R leads satisfies the telephone station disconnect requirements and hence the telephone call is dropped.
While the likelihood of a hold condition request being initiated during the OSI period is small, the proliferation of automatic answering devices may enhance the likelihood. Thus, the elimination of this possibly troublesome anomaly in the telephone circuit operation is important.
Prior art approaches to enabling a hold condition during an OSI condition include temporarily bridging, during the OSI, an auxiliary central office battery supply voltage across the T and R leads, altering the central office equipment, or changing central office switch timing to eliminate the OSI interval. These central office type solutions require extensive changes and are overly complicated and expensive.
Thus, it is a continuing quest to solve the problems of establishing a hold condition during OSI conditions without changes in the central office or without using complicated and expensive central office equipment.